linux-kernel - Re: [PATCH RFC 0/4] change sb_writers to use percpu_rw

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Date:	Fri, 17 Jul 2015 11:27:42 +1000
From:	Dave Chinner <david@...morbit.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Jan Kara <jack@...e.cz>, Dave Hansen <dave.hansen@...ux.intel.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Daniel Wagner <daniel.wagner@...-carit.de>,
	Davidlohr Bueso <dave@...olabs.net>,
	Ingo Molnar <mingo@...hat.com>, Tejun Heo <tj@...nel.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC 0/4] change sb_writers to use percpu_rw_semaphore

On Thu, Jul 16, 2015 at 07:32:56PM +0200, Oleg Nesterov wrote:
> 
> 	#ifdef CONFIG_LOCKDEP
> 		/*
> 		 * We want lockdep to tell us about possible deadlocks with freezing but
> 		 * it's it bit tricky to properly instrument it. Getting a freeze protection
> 		 * works as getting a read lock but there are subtle problems. XFS for example
> 		 * gets freeze protection on internal level twice in some cases, which is OK

Sorry, I've missed something here - where is XFS nesting
sb_start_intwrite() calls?  XFS only has a call to
sb_start_intwrite() in xfs_trans_alloc() and an open coded
equivalent in xfs_iomap_write_unwritten(). However, we cannot create
nested transaction contexts as doing so is *guaranteed to deadlock
the journal*. (e.g. why we use PF_FSTRANS to trigger GFP_NOFS
allocation as it prevents direct memory reclaim from causing nested
transactions.) Hence if we taking multiple FREEZE_FS level
references at a time then there is a bug that needs fixing....

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com
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